The invention relates to optics, and particularly concerns an optical beam expander.
Known is an expander of an optical beam propagating in a planar optical waveguide, which expander includes a waveguiding lens (see, e.grating. Y. Abdelrezak, Chen S. Tsai and T. Q. Vu. An Integrated Optic RF Spectrum Analyzer in ZnOxe2x80x94GaAsxe2x80x94AlAsAs Waveguide, J. Lightwave Technology, 1990, Vol. 8, No. 12, pp. 1833-1837). The device operates similarly to a standard light beam collimator implemented on a volume lens. A small-size optical radiation source is positioned in focus of a planar lens, the light from which source first is expanded due to the diffraction divergence, and then the light is collimated by the planar lens. Irrespective of a possible type of the used waveguiding lenses (Fresnel, Luneberg, geodetic lenses, etc.), this device is characterized with large dimensions due to both geometric sizes of lenses themselves and a great length (not less than focal distance of lenses themselves) that is required to form a sufficiently broad and well-collimated light beam.
Also known is a beam expander (V. Neumann, C. W. Pitt, L. M. Walpita, Guided-wave holographic grating beam expanderxe2x80x94fabrication and performance, Electronics Letters, 1981, v. 17, No. 4, p. 165-166), wherein an optical beam in a waveguide is expanded using the Bragg diffraction grating. This device includes a planar optical waveguide and a beam expanding means in the waveguide plane, which means is disposed on the path of the radiation beam and is a diffraction grating having a very small spacing, grooves of which grating are implemented on the optical planar waveguide surface at the angle that is equal to Bragg angle (xcex8B) as measured in respect of the incident light beam:
sinxcex8B=K/2kxe2x80x83xe2x80x83(1)
where K=2xcfx80/xcex9, xcex9 is diffraction grating spacing, k=2xcfx80 N/xcex0 is constant value of propagation of light in a waveguide, xcex0 is light wavelength in vacuum, N is effective refractive index of the guided mode of the optical waveguide.
This device is capable of providing deflection by 90 degrees of a narrow (less than 1 mm) light beam that is incident onto the diffraction grating at Bragg angle. Width of the expanded beam can be 5-10 mm. The width depends on parameters of the interacting waves and diffraction grating. In the known device, the diffraction grating on a glass waveguide was manufactured by the method of ion etching through a photoresist mask illuminated by a holographic technique by superposition of two optical beams. The grating had spacing of 0.6 xcexcm and was 0.3 nm deep. That approach provided the diffraction efficiency of 16% for the guided mode, with effective refractive index of 1.536 at the helium-neon laser wavelength.
But the discussed device has the following disadvantages: the high divergence of the outputted light beam, which divergence is determined by divergence of the incident light beam having a narrow aperture, as well as the spatial inhomogeneity of the expanded beam, which inhomogeneity is caused by the technical difficulty of fabrication of diffraction gratings of sub-micron sizes on a large aperture. This limitation is a principal one in terms of the practical use of a beam expander in acousto-optical (AO) devices for processing and transmitting data, e.g. AO spectrum analyzers, tunable filters, etc. In such devices, the optical beam divergence determines such important parameter as the number of resolvable spots.
The invention is basically directed to the object of developing of a beam expander that will have the minimal dimensions and, simultaneously, a low divergence of the outputted optical radiation.
Said object is to be solved as follows: in a radiation beam expander that includes a planar optical waveguide and a beam expanding means in the planar waveguide plane, which means is disposed on the path of the radiation beam: according to the inventionxe2x80x94the beam expanding means is implemented in the form of a stripe waveguide provided with a set of unit reflectors that overlap its aperture, and which is disposed in the planar waveguide plane either within said waveguide, or in the vicinity thereof, with providing of possibility of transition of the radiation beams, reflected by the unit reflectors, into the planar waveguide, the inclination angle and relative position of the unit reflectors being selected such that the phase difference at the operating radiation wavelength, for any pair of beams reflected from different unit reflectors, is essentially multiple of 2xcfx80.
To ensure an essential suppression (over 20-30 dB) of sidelobes of the outputted optical radiation""s directivity pattern, the unit reflectors are advantageously implemented as having different reflectance, whose value diminishes from the middle portion of a stripe waveguide to its ends.
In the preferable embodiment, angle of inclination of the unit reflectors with respect to the longitudinal axis of a stripe waveguide is selected to be essentially 45xc2x0.